1. Technical Field
The invention relates to printers and printing. More particularly, the invention relates to a printer having a global skew adjustment and a print head assembly having an adjustment mechanism.
2. Description of the Background Art
Various problems arise in connection with modern printers. For example, problems are encountered in such printers with regard to skew adjustment and print head assembly adjustment. The following discussion addresses these problems.
Global Skew Adjustment
Web printing involves the use of rolls (or “webs”) of paper or other substrate. In a narrow web, single pass digital printing application, typical image printers support printing in any of a number of colors, e.g. 4 , 6, or 8 colors. Each of these colors may have numerous of inkjet print heads that are organized in different arrays, lines, and columns (see FIG. 1). As a result of this arrangement, in the print zone 11, the distance between the first color 10 and last color 12 can be relatively large. For example, in the Jetrion 4900 the distance between first and last color is about 600 mm. For best printing quality the print heads should be perfectly adjusted. Each color should be perpendicular to the web motion 13 and all the colors should be able to print in the same spot.
Currently, digital printer manufacturers use different techniques to adjust the positioning of the heads. Only properly adjusted systems can print with high quality.
As shown in FIG. 2, in single pass printer the print heads are stable as the web 20 moves past the print heads. In a typical printer, the web is advanced under the print zone 22. To adjust the print heads properly, it is necessary to take into consideration the fact that the web is moving in a straight direction. Based on this, every head must be moved and calibrated in the cross and skew directions.
After a final adjustment, every web should travel past the heads exactly as if it had been placed on the printer during initial calibration. In reality (see FIG. 3), however, different materials (webs) from different vendors, or even different batches of webs from the same vendor, could move in a slightly different manner under the print zone. Many factors such as tension inside the roll or different thickness across the web could steer the web a little bit one way or the other and, as a result, a single color may not be perpendicular to the motion of the web, such that the colors do not align properly.
Different companies use different techniques to try to fix this issue:
As shown in FIG. 4, some printer manufacturers readjust the cross and skew of the heads 40 to accommodate the new web directionality (α). While this approach can produce the desired quality, the process is complicated and the user must repeat the adjustment procedure every time a new roll of web material is used.
As shown in FIG. 5, other printer manufacturers try to correct the web steering. Their target is to bring the web to default directionality 13. In such cases the exit roll 53 is inclined after the print zone. This brings the web to right direction, but generates different tensions across the web, where one side of the web has a higher tension 52 and the other side of the web has a lower tension 50. Not every material moves smoothly under these conditions, which can generate wrinkles and plastic deformation to the materials.
Print Head Assembly Adjustment
Industrial printers contain many print heads. FIG. 6 is a schematic drawing showing a multiple print head arrangement. The Jetrion 4900-330, for example, has up to 25 print heads. Different vendors use different print heads, but their systems also contain a large number of heads as well. Every head has hundreds of nozzles, usually organized in rows.
The distance between neighbor nozzles and between rows within a single print head is very accurate (see “a” on FIG. 6). However, while the heads themselves are made very accurately, to make an array of several print heads and maintain the printing quality is very challenging task. First, it is necessary to place the individual head so that the nozzles rows are perpendicular to the web 90 (see 90° on FIG. 6). Next, it is necessary to place the other heads from the same color group in a cross direction so that the distance between last nozzle of first head 92 and first nozzle of the second head 94 is equal to the distance between nozzles in the head itself (see “a” on FIG. 6). To achieve head placement accuracy, every single head should be adjustable in the cross and skew directions.